Malnutrition (Undernutrition) — full guide (definitions, grading, diagnosis cut-offs, primary vs secondary causes, and management)

Mayta
Jan 25
5 min read

Updated: Jan 26

1) Core definitions

Malnutrition = deficiency, excess, or imbalance of nutrients, or impaired nutrient use. (TB Knowledge Sharing) Clinically, people often mean undernutrition (the “too little” side):

Undernutrition includes (TB Knowledge Sharing)

Wasting / Acute malnutrition = recent/rapid weight loss → low weight-for-height/length and/or nutritional oedema. (fscluster.org)
Stunting / Chronic malnutrition = long-term growth failure → low height-for-age. (World Health Organization)
Underweight = low weight-for-age (a mix of acute + chronic) (apps.who.int)
Micronutrient deficiencies (iron, vit A, iodine, etc.)

✅ Acute vs chronic = pattern/time-course ✅ Primary vs secondary = cause (explained below)

2) “Secondary Acute Malnutrition” — what it means

Secondary acute malnutrition = wasting/SAM/MAM caused by an underlying illness (not just lack of food). Examples:

chronic infection (TB/HIV), congenital heart disease, CKD, IBD/celiac, malignancy
malabsorption, chronic diarrhea
increased metabolic demand (sepsis, burns)
feeding problems (cleft palate, neurodisability), medications

Primary acute malnutrition = mainly due to inadequate intake/food insecurity or poor feeding practices (with no major medical driver).

⭐ There isn’t a widely used “tertiary malnutrition” category in standard WHO-style classification. If someone says “third,” they usually mean mixed (primary + secondary) or acute-on-chronic (wasting on top of stunting).

3) Diagnostic criteria & grading (cut-offs)

A) Children <5 years (most tested + most used programmatically)

Moderate acute malnutrition (MAM) (TB Knowledge Sharing)

WHZ/WLZ (weight-for-height/length z-score): ≥ −3 to < −2, and/or
MUAC: ≥115 mm to <125 mm
No nutritional oedema

Severe acute malnutrition (SAM) (TB Knowledge Sharing)

Nutritional oedema and/or
WHZ/WLZ < −3 and/or
MUAC <115 mm

Chronic malnutrition (Stunting) (World Health Organization)

HAZ < −2 = stuntedCommon clinical grading (widely used in surveys):
Moderate stunting: HAZ < −2 to ≥ −3
Severe stunting: HAZ < −3 (common DHS/UNICEF convention)

Underweight (WAZ) (often used in growth monitoring)

WAZ < −2 = underweight
WAZ < −3 = severe underweight (commonly used in UNICEF/MICS materials)

⚠️ “Mild wasting” is not a standard WHO program category. Some hospitals use “mild/moderate/severe” by z-score (e.g., −1 to −2 as mild), but in public health CMAM you mainly see MAM vs SAM, plus “at risk” groups.

B) Children & adolescents 5–19 years (TB Knowledge Sharing)

Use BMI-for-age Z-score:

Moderate: ≥ −3 to < −2
Severe: < −3

C) Adults >19 years (thinness grading) (TB Knowledge Sharing)

Mild thinness: BMI 17.00–18.49
Moderate thinness: BMI 16.00–16.99
Severe thinness: BMI <16
Underweight: BMI <18.5 (TB Knowledge Sharing)

4) How to diagnose in practice (stepwise)

Step 1 — Confirm the type (acute vs chronic vs both)

Measure:

Weight, length/height, calculate z-scores if possible
MUAC (6–59 months)
Check bilateral pitting oedema (nutritional oedema)

Step 2 — Grade severity using the cut-offs above

✅ classify as MAM or SAM (or stunted/underweight)

Step 3 — Decide Primary vs Secondary

Suggestive of secondary (disease-related):

adequate food access but weight loss continues
chronic diarrhea, persistent fever/cough, recurrent infections
poor feeding mechanics, vomiting, dysphagia
organ disease signs (cardiac murmur, edema not typical nutritional, hepatosplenomegaly)

5) Management overview (separate OPD vs IPD)

A) OPD / Community (Uncomplicated MAM & many uncomplicated SAM)

1) OPD management — MAM

Definitive

Supplementary feeding (program dependent): fortified blended foods / RUSF + diet counselling (energy + protein dense foods)
Treat/stop ongoing losses: diarrhea plan, deworming per local protocol, catch-up immunizations

Supportive

Feeding counselling: frequent meals, continued breastfeeding, safe water/WASH
Screen caregiver depression, neglect, food insecurity → social support

Monitoring

Weekly/biweekly weight/MUAC
Danger signs → refer IPD

2) OPD management — Uncomplicated SAM (CMAM style)

Key decision = appetite + complications.A commonly used appetite approach uses RUTF test; if child eats adequately and is clinically stable → outpatient; otherwise inpatient. (National Department of Health)

✅ OPD if:

no danger signs/complications
passes appetite test
caregiver can follow weekly follow-up

❌ Send IPD if:

failed appetite test, lethargy, shock, severe dehydration, hypoglycemia/hypothermia, severe anemia, pneumonia/sepsis, or significant oedema (program rules vary, but oedema is a red flag)

Definitive

RUTF per weight (local program table)
Treat infections per protocol, deworming where appropriate

Follow-up

Weekly: weight gain, oedema check, MUAC, compliance, intercurrent illness

B) IPD / Hospital (Complicated SAM, oedema, failed appetite, danger signs)

IPD management Complicated SAM (WHO “10-step” style)

1) Immediate stabilization (first 24–48h)

✅ Hypothermia

Feed immediately then every 2–3 hours and keep warm; kangaroo technique in infants. (National Department of Health)

✅ Dehydration (high-risk of overhydration)

Avoid IV rehydration except shock. Rehydrate slowly PO/NG with ORS ~20 ml/kg/hr for 4h, reassess frequently. (National Department of Health)
If shock/failed PO/NG: carefully selected IV fluids may be used with close monitoring. (National Department of Health)

❌ Do NOT “rush fluids” like a typical dehydration case—risk of heart failure/overhydration is real. (National Department of Health)

✅ Electrolytes

SAM commonly has K/Mg deficiency and excess total body sodium.❌ No diuretics for oedema; high sodium loads can be dangerous. (National Department of Health)

✅ Treat infection (assume infection even if no fever)

Example inpatient regimen from WHO pocket book:
- Benzylpenicillin 50,000 units/kg IM/IV q6h + Gentamicin 7.5 mg/kg once daily for ≥5 days (National Department of Health)
- If septic shock / very severe infection / pneumonia: add Ceftriaxone 80 mg/kg once daily (often 5–10 days) (National Department of Health)

2) Feeding in stabilization

✅ Start F-75 (starter feeds): small, frequent (q2–3h) to prevent heart failure/refeeding problems (program protocols).

3) Transition & rehabilitation

✅ Gradual transition F-75 → F-100 or RUTF over 2–3 days as tolerated; target high kcal/protein for catch-up growth. (National Department of Health)

4) Micronutrients (important “don’t do” rules)

✅ Vitamin A

If using premixed therapeutic feeds, don’t give high-dose vitamin A routinely unless eye signs or measles history; therapeutic foods already contain it. (National Department of Health)
If indicated:
- <6 months: 50,000 IU
- 6–12 months: 100,000 IU
- ≥12 months: 200,000 IU (National Department of Health)

✅ Iron

Do NOT give iron in stabilization phase. Start ~3 mg/kg/day only after moving to catch-up feeding; and no extra iron if on RUTF (already contains enough). (National Department of Health)

5) Discharge & follow-up criteria (very testable)

✅ Discharge from nutrition program only when:

WHZ/WLZ ≥ −2 and no oedema ≥2 weeks, OR
MUAC ≥12.5 cm and no oedema ≥2 weeksUse the same indicator as admission; don’t use % weight gain as discharge criterion. (National Department of Health)

6) “Primary vs Secondary” — management differences

Primary (food insecurity / poor feeding)

✅ Definitive

Nutritional rehabilitation (RUTF/RUSF/food-based catch-up)
Household food support + social protection
Caregiver feeding skills, breastfeeding support, WASH

✅ Supportive

Routine immunizations, deworming where indicated, manage diarrhea/ARI promptly

Secondary (disease-related)